A District police officer, accused of accepting sex from a woman rather than arresting her for prostitution, testified yesterday that he did not have sex with the woman and that he believed the woman was angry with him because he forced her to return money to one of her customers.

Albert J. Sell told a D.C. Superior Court jury that after he ordered the woman from a pickup truck on Sept. 30, 1980, he told her to return some money her customer accused her of taking from the dashboard.

Sell said the woman then became so boisterous and belligerent that his "main concern" was to avoid conflict and leave.

"The woman kept cursing at me . . . , " said Sell, describing how Lois Frontuto, yelling, followed him to his police cruiser. "I just wanted to get away from her."

Sell said he did not arrest Frontuto that night because he had not seen her "perform any services."

Prosecutor Mark Dubester has alleged that Sell, charged with bribery and sodomy, took Frontuto to an empty parking garage early that morning and had her perform a sex act on him in exchange for not arresting her.

Several months later, Dubester alleged, Sell attempted to block Frontuto from testifying before a grand jury after she filed a complaint. Sell has been charged with obstruction of justice in connection with that incident.

Yesterday, Sell said he did not intend to threaten Frontuto and that the reason he told her she did not have to testify before the grand jury was that he was convinced she would not "go down and tell them the truth."

In March 1981, Frontuto agreed to wear a police tape recorder and arrange to meet Sell. In a taped account of that meeting played during the trial last week, Sell urged Frontuto not to testify and warned her that if he were convicted, other police officers would lock her up "right and left."

"I couldn't believe it," Sell said of the time Frontuto told him she had received a subpoena to appear before the grand jury. "I was sick . . . . I believed Lois Frontuto would lie to the grand jury."

When asked by Dubester if his intentions that March day were to avoid trial, Sell answered, "Yes, sir."

"Mr. Dubester, I don't want to be sitting here" for having done his job as a police officer, Sell said.

During the past week, Sell has sat at the defense table, making notes on a yellow legal pad or twirling a pen. After his testimony yesterday, he bowed his head to his hands. He left the courthouse holding hands with his wife.

Defense attorney Robert E. Greenberg has argued during the trial that Frontuto, a convicted prostitute, is a vindictive woman who was determined to make complaints against police officers to scare them so they would not bother her. Sell, he has said, was an aggressive policeman who was the unfortunate victim of Frontuto's wrath.

In testimony yesterday morning, Greenberg called as a defense witness a police officer who was present Sept. 30 while Frontuto complained to another officer that Sell had her perform a sex act on him. The officer said Frontuto screamed at him that if she did not "get" Sell, she would get him.

Also testifying yesterday for the defense was the owner of Sholl's cafeteria, who described Sell's reputation as "top-notch" among the merchants on Sell's patrol beat.

Sgt. Robert Gannon, one of Sell's superiors, said he often told Sell he was spending too much time patrolling the busy prostitution corridor of Thomas Circle, away from his beat in the business section of town.

However, he also testified that many police officers acted similarly because the business patrol beat was "boring" after about 2:30 a.m. and prostitutes often gave police officers other information.

He characterized Sell as an aggressive officer "who made some damn good cases . . . . He didn't like the boredom. It was not [his] personality."